[ENSCORESW-664]. Adding support for multiple variation databases from the Slice object.

This work allows for the hosting of more than one variation database (similar to
core-like databases such as rnaseq or otherfeatures). The Slice methods for core objects
allowed the specification of the database type. Variation methods now have the same
option. We have pushed much common logic into 4 methods to provide this automatic
defaulting behaviour.